Depth of field in the field

Point of focus and depth of field choices will change how you look at an image.

Take a minute to examine the three above photos.  They were all shot from the same position with the same amount of available light.  In each one though there is something different.  In the first one (far left) you can see that the pillars are in focus but the church windows are not.  The middle one is reversed with the pillars being out of focus and the church being in focus.  The last one (far right) has both in focus at the same time.  There was no photoshop magic going on here; it all has to do with how the camera is set up.

There are three different parameters which were altered to create the different images.  The first involves aperture setting; that's the "iris and pupil" of the camera lens.  Once it is set the second parameter, shutter speed, falls into place.  In the case of the first two images that is f/5 at 1/200th of a second (ISO was constant in all three being set at 100).  The third image used an aperture of f/14 at 1/20th of a second.

Both the aperture and shutter speed (including ISO, but don't worry about that now) are responsible for the exposure component of the shot.  The f/5 aperture setting lets in a lot of light but also gives very little depth of field.  Since the aperture is open so wide, the shutter speed has to be faster to give the correct exposure.  The f/14 aperture is much smaller, letting in much less light; at the same time though it gives more depth of focus to the photo.  Since there is so much less light a longer shutter speed is needed to provide the same exposure, which is why the last shot has a shutter speed of 1/20th of a second.

The third parameter is point of focus.  Since the first two shots use low apertures causing little depth of field, the lens is focused on the pillars (first) or the church (second) causing the other components of the shot to be blurry.  It is interesting how focus will help draw your eye; the way you look at an image is partly based on what is in focus.  In the third photo the point of focus is about a third of the way to the church from the pillars.  Greater depth of field means there is more in focus in the shot; in this case it is just barely enough to allow both parts of the scene to be in focus.

Playing with aperture, shutter speed, and point of focus will allow you to develop an understanding of how these things work.  As you play with them more and more you will get better at it.  The basis of this is one of the cornerstones of photography.